A conventional approach to acquiring high quality iris or face images of moving subjects is to freeze the subject motion by using extremely short exposures. The subject must be brightly illuminated by a flash in order to get a well exposed image. This approach commonly breaks down for larger distances because the flash power that is required in order to obtain an acceptable image often becomes eye unsafe.
Producing a well exposed image without flash illumination typically requires extending the image exposure, which degrades the image quality. One drawback with this approach is that extending the image exposure typically introduces motion blur unless the relative motion between the subject and the camera sensor is eliminated.
The motion of a subject relative to the camera sensor can be real, apparent or both. Real motion is the result of the physical motion of the subject and/or the sensor. Real motion is described by a velocity vector which gets decomposed into two components. The axial velocity vector points toward the sensor and is aligned with the optical axis of the sensor. The lateral velocity vector is the velocity vector projection into an X-Y plane perpendicular to the optical axis. Axial velocity affects the focusing of camera optics and may introduce magnification blur for long exposures. Lateral velocity causes motion blur.
Moveable lenses are widely used as image stabilizing elements in telephoto objectives for photographic cameras. One known approach is to use a pair of inertial sensors inside the objective to detect camera motion resulting from the user's unsteady hand and then move the stabilization lens during exposure to compensate for it. An alternative approach employs a tip-tilt mirror to compensate for the relative motion. This approach is applicable only to scenarios in which all motion is due to that of the camera, because sensors cannot be installed on the moving subject that is imaged.
One drawback with these types of solutions is that they typically cannot track very fast motions. In addition, these approaches usually utilize delicate mechanical parts that make their use problematic in military and security applications.